Heat wave sweeps across the U.S.

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Unhealthy levels of heat and humidity are encompassing much of the eastern half of the U.S., according to NOAA's National Weather Service, as a persistent heat wave continues its grip on the central U.S. while expanding into the East.

According to NOAA's National Weather Service, approximately 132 million people in the United States are under a heat alert (Excessive Heat Warning or Watch or Heat Advisory) as of Friday morning.

Temperatures in the 90s to near 100 degrees will feel as hot as 115 degrees or higher when factoring in the high humidity. Record high temperatures are likely to be set in some locations — adding to the more than 1000 records that have been set or tied so far this month.

“This heat is dangerous on many levels,” said Jack Hayes, director of the National Weather Service. “Temperatures and humidity levels are high, the heat will be prolonged, and very warm temperatures overnight won’t provide any respite. All of these factors make this an unhealthy situation.”

Satellites 'seeing' humidity across the U.S.

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High humidity along with hot temperatures is making this week's heat wave in the central and eastern U.S. especially unhealthy. The dew point temperature is a measure of how much moisture is in the air. This animation shows average daily dew point data from the North American Model (NAM) model from July 18 through July 24, 2011.

Heat wave sweeps across the U.S.

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A shroud of high pressure has taken a foot-hold over the U.S. from the Plains to the Northeast, and with it has brought temperatures well into the 90's and 100's for half of the country. This animation shows the predicted daily high temperatures from NOAA's high resolution North American Model (NAM) from July 13-21, 2011.